As is well known, polyester containers as represented by polyethylene terephthalate (PET) containers have been used in a variety of applications and, most, widely, for foods and beverages. Further, cup-like containers having a wide mouth and a flange along the circumferential edge of the mouth have been placed in practical use as thermoplastic resin containers for foods and beverages.
As a method of producing the cup-like containers, a patent document 1 teaches a method of producing a cup-like container by forming a sheet-like preform by injection forming and by subjecting the preform to the heat forming such as plug-assisted forming.
According to the above method of forming the preform by injection forming and heat-forming the preform, a problem remains in that a central portion of the bottom of the container is formed thick due to the remaining portion of the injection gate, and breakage easily occurs due to the thick portion. A patent document 2 proposes a production method which avoids the formation of a thick portion. Concretely speaking, the patent document 2 teaches a plug-assisted forming method in which a portion corresponding to the center of the bottom of the preform of which the peripheral edge is fixed is held between a preforming plug and a bottom-holding plug, and the preforming plug is extruded in this state to form the preform.                Patent document 1: JP-A-5-69478        Patent document 2: JP-A-6-134850        